Many different types of data storage media have been developed to store information. Traditional media, for instance, include magnetic media, optical media, and mechanical media. Increasing data storage density is a paramount goal in the development of new or improved types of data storage media.
Holographic data storage is an emerging technology, that provides many advantages over traditional forms of data storage. One advantage of holographic data storage is that it enables volumetric storage of digital data in holographic storage media. The volumetric storage is accomplished by making use of a full thickness of the holographic storage media, thereby providing data densities proportional to thickness of the holographic storage media. More specifically, the recording of a digital bit is distributed throughout a recording volume of the holographic storage media, rather than as a localized region of magnetization or optical change. Moreover, each image may contain many bits, and many images can be uniquely recorded into the storage media and extracted from a finite volume of the storage media. This makes possible capacities of more than 1,000 GB on a CD disk format. By comparison, DVD technology provides only 9 GB on a single-sided disk.
Another advantage of holographic data storage is that it represents an opportunity to significantly increase data transfer rates well beyond those that may be achieved with conventional DVD technology. More specifically, in holographic data storage, data is transferred as pages of optical information from a single head. This contrasts with conventional DVD technology, which transfers data in a serial stream of bits. Consequently, holographic data storage provides a substantially faster data transfer rate from the single head, surpassing 100 MB/sec. By comparison, conventional DVD technology provides a data transfer rate of only 5 MB/sec.
In holographic data storage, data is stored within the holographic storage medium as an interference pattern resulting from the interference of object and reference beams. More specifically, the object beam is encoded with the data using a spatial light modulator that selectively blocks the beam or allows the beam to pass through, thereby creating a pattern of light and dark regions. The object beam including the pattern is then projected onto a spinning disk including the holographic storage medium. A reference beam that is coherent with the object beam is also projected onto the spinning disk. The reference beam interferes with the object beam, thereby forming the pattern of the object beam in the holographic storage medium.
Photopolymer-based holographic data storage applications are one example of holographic data storage applications. In the photopolymer-based holographic data storage applications, the holographic storage media includes a binder which is generally a non-polymerizable component, a polymerizable monomer, and a photoacid generator. As a result of the interference created by the reference and object beams, the photoacid generator initiates curing or polymerization of the polymerizable monomer in the light regions of the pattern. The binder typically exhibits compatibility with the polymerizable monomer prior to curing of the polymerizable monomer, yet diffuses from the polymer of the polymerizable monomer after curing. As a result, the polymer of the polymerizable monomer localizes in the light regions of the pattern, and the binder localizes in dark regions of the pattern.
The binder has a different refractive index from the polymer of the polymerizable monomer. As a result, the regions including the polymer of the polymerizable monomer exhibit a different refractive index than the regions including the binder. The difference in refractive index between the regions provides a refractive index modulation that is needed to form a hologram of the pattern in the holographic storage medium, with larger differences between the respective refractive indexes resulting in greater storage capacity of the holographic storage media. As such, it is important that a refractive index of the binder be as high as possible.
Recently, it has been found that certain epoxy monomers, when used as the polymerizable monomer, minimize problems with shrinkage that exist when other polymerizable monomers are included in the holographic storage media. Specifically, the epoxies exhibit minimal shrinkage after polymerization, which is desirable for holographic data storage applications.
To obtain holographic storage media with maximum storage capacity and image fidelity, it is important to minimize light scattering before curing, after partial cure, and after complete cure of the polymerizable monomer. To minimize the light scattering, it is important that the binder is compatible with the polymerizable monomer before curing, but diffuses during curing of the epoxy monomers to result in domains rich in the epoxy monomer or polymer thereof, and other domains rich in the binder. More specifically, during polymerization, the polymerizable monomer diffuses to monomer rich domains and reacts to form polymers. After reaction, the domains are rich in polymer. The diffusion of the monomer toward the domains rich in the polymer results in diffusion of the binder away from the domains rich in the polymer, thereby resulting in the domains rich in the polymer and the domains rich in the binder. The diffusion results in the necessary index modulation as described above.
In the holographic storage media including the epoxy monomers, binders that have found particular use include, in particular, poly(methyl phenyl siloxanes) and oligomers thereof, such as 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane. Further, other binders that have found particular use include polysiloxane chains having electroluminescent side groups, which comprise a plurality of aromatic rings, as disclosed in U.S. Pat. No. 5,414,069 to Cumming et al. (hereinafter the '069 patent). The polysiloxanes of the '069 patent may exhibit high refractive indexes. Although the poly(methyl phenyl siloxanes) are compatible with the epoxy monomers and diffuse during curing of the epoxy monomers to result in the domains rich in the epoxy monomer or polymer thereof, and other domains rich in the binder, and although the polysiloxane chains of the '069 patent may exhibit high refractive indexes, there remains an opportunity to provide a siloxane that exhibits the same compatibility and diffusion properties as the poly(methyl phenyl siloxanes) but that has a higher refractive index, even higher than the polysiloxanes of the '069 patent, to minimize light scattering and maximize data storage capacity of the holographic storage media.